Process for the preparation of esters of carbonic acid

ABSTRACT

A process is described whereby carbonic acid esters, i.e., alkyl, aryl and cycloalkyl carbonates, are prepared by reacting an alcohol with oxygen and carbon monoxide in the presence of a catalyst selected from copper complexed with an organic molecule.

The present invention refers to a process for the preparation of estersof carbonic acid. Esters of carbonic acid are known which are useful assolvents and polymerization agents in the production of polycarbonatesby transesterification with glycols and diphenols. Alkyl, alkyl-aryl oraryl carbonates are employed in this case.

According to the known art the above esters are obtained by reacting analcohol or a glycol with phosgene or chloroformiates in the presence ofbases selected from the group consisting either of hydroxides, alkalineor alkaline-earth carbonates, or of pyridine or other organic bases. Inthe first case the reaction is carried out in the presence of solventsand at a controlled temperature in order to get the final products outof the hydrolysis by the same bases and the water anyhow present in thereaction vessel.

We have now found it possible to prepare esters of carbonic acid byreacting the alcohol for esterification with carbon monoxide and oxygenin the presence of suitable catalysts. The reaction goes on up toquantitative yields with respect to the fed reactants.

The process according to the present invention may be substantiallyschematized by the following way: ##STR1## wherein R is a hydrocarbonradical selected from the alkyl, aryl or cycloalkyl radicals.

The reaction is catalyzed by complexes of metals (M) of the 1st B, 2nd Band 8th group of the periodic system. Very suitable metals are Cu, Ag,Au, Zn, Cd, Hg, Fe, Co, Ni. i.e., metals able to exist in two differentvalence conditions by means of redox reactions. The corresponding metalions are able to carry out the above reaction only with very low yieldsand through very unfavourable kinetics. Very suitable catalysts are thecomplex molecules having the formula MX_(n) L_(m) wherein n rangesbetween 1 and 3, m ranges between 5 and 3. X is an anion and L is aneutral ligand. The complex may be a saline complex too as [ML_(m) ]⁺^(n) n X⁻. The n + m sum is generally equal to six. i.e. the complexesmay exist at the most in a hexacoordination state, which may dissociatein solution. The more suitable anions are halide ions, CN, C10₄ ⁻ ,complex anions as BF₄ ⁻ and the like. The L ligands are selected fromthe group consisting of organic bases as pyridine, dipyridyl, imidazole,phenanthroline, alkyl or aryl phosphines, dimethylsulfoxide,dimethylformamide, quinuclidine, CO; suitable ligands are also thenitriles as CH₃ CN, C₆ H₅ CN, and the bidentate ligands as malonitrile,succinodinitrile, adiponitrile and the like.

The inventive reaction will be illustrated by the unrestrictiveemployment of complexes of Cu, Co and Au. It is preferable that thecomplexes contain the aforesaid metals in their lower oxidation state;from a kinetic point of view it is interesting to use the carbonylcompounds obtained from the above complexes; they are prepared beforebeing introduced into the reaction vessel and then are put therein.

The reaction is carried out in a solvent consisting either of alcohol tobe oxidized or of a mixture of the alcohol with an inert solvent. Thelatter is selected with the view of removing of the reaction water bymeans of an azeotropic distillation. Therefore the more suitablesolvents are C Cl.sub. 4, C₆ H₆, CH₃ --C₆ H₅ and the like.

The complex may be obtained by starting from the metal ion dissolved inthe solvent to which a stoichiometric amount of the complexing base isadded. It is generally prepared in advance, characterized and thenintroduced into the reaction vessel.

The inventive process may be carried out in a wide range of pressuresand temperatures and conveniently runs out of atmospheric pressure androom temperature too. Both factors may be usefully changed in order toadvantageously influence the reaction kinetics and their limit valuesdepend only on the boiling temperature of the mixture or on the firmnessof the introduced catalyst. The temperature generally ranges between-20° C. and +110° C. and preferably between 50° and 70° C. The inventivereaction satisfactorily runs at room pressure, but it is preferable toincrease the partial pressure of CO to values higher than theatmospheric one up to 3 + 4 atmospheres. This fact kinetically favoursthe reaction owing to the stabilization of the carbonyl derivativeswhich are the process catalysts.

The invention is now illustrated by the following unrestrictiveexamples:

EXAMPLE 1

C10.424 g of Cu₂ Cl₂ were dissolved into 25 cc. of methyl alcohol and 25cc of pyridine. The obtained solution was oxidized by O₂ at roompressure and temperature, then was put into a CO atmosphere at oneatmosphere pressure and at a temperature of 25° C. After the carbonmonoxide has stoichiometrically been absorbed, the solution wassubjected to gas-chromatography analysis.

An amount of 0.370 g of CO(OCH₃)₂ was found by means of agas-chromatography comparison with standard samples; the said amount wasequal to 96.2% with respect to Cu. The yield was quantitative.

Three other following cycles of O₂ and CO absorption were carried out onthe same solution:

    ______________________________________                                        II        cycle     CO(OCH.sub.3).sub.2                                                                        g. 0.730                                     III       cycle     CO(OCH.sub.3).sub.2                                                                        g. 1.080                                     IV        cycle     CO(OCH.sub.3).sub.2                                                                        g. 1.450                                     ______________________________________                                    

Neither by-products nor CO₂ were observed in the discharge gasesobtained by all the above operations.

EXAMPLE 2

0.378 g of Cu₂ Cl₂ were dissolved into 25 cc of ethyl alcohol, 2.41 g ofdipyridyl being present. The obtained solution was oxidized by O₂ atroom temperature and pressure, then it was put into a CO atmosphere atone atmosphere pressure and at a temperature of 25° C. After the carbonmonoxide has stoichiometrically been absorbed the solution was subjectedto gas-chromatography analysis. An amount of 0.440 g of CO(OC₂ H₅)₂ wasfound by means of a gas-chromatography comparison with standard samples;the said amount was equal to 97.5% with respect to Cu. The yield wasquantitative.

Two other following cycles of O₂ and CO absorption were carried out onthe same solution:

    ______________________________________                                        II        cycle     CO(OC.sub.2 H.sub.3).sub.2                                                                 g. 0.870                                     III       cycle     CO(OC.sub.2 H.sub.3).sub.2                                                                 g. 1.315                                     ______________________________________                                    

Neither by-products nor CO₂ were observed in the discharge gasesobtained by the above operations.

EXAMPLE 3

0.400 g of Cu₂ Cl₂ were dissolved into 25 cc of benzyl alcohol, 3.20 gof ortho-phenantroline being present. The obtained solution was oxidizedby O₂ at room temperature and pressure, then it was put into a COatmosphere at one atmosphere pressure and at a temperature of 25° C.After the carbon monoxide has stoichiometrically been absorbed thesolution was subjected to gas-chromatography analysis.

An amount of 0.970 g of benzyl carbonate was found by means of agas-chromatography comparison with standard samples; the said amount wasequal to 96% with respect to Cu. The yield was quantitative.

Two other following cycles of O₂ and CO absorption were carried out onthe same solution:

    ______________________________________                                        II       cycle     CO(OCH.sub.2 C.sub.6 H.sub.5).sub.2                                                          g. 1.930                                    III      cycle     CO(OCH.sub.2 C.sub.6 H.sub.5).sub.2                                                          g. 8.900                                    ______________________________________                                    

Neither by-products nor CO₂ were observed in the discharge gasesobtained by the above operations.

EXAMPLE 4

0.085 g of Cu₂ Cl₂ were dissolved into 3 cc of anhydrous pyridine. Thesolution was put into a CO atmosphere up to the absorption of almost astoichiometric volume of carbon monoxide, by checking the appearance ofthe carbonyl band at 8080 cm.sup.⁻¹ by means of I.R. examination. Thesolution was then diluted by 25 cc of ethyl alcohol and subjected to theaction of a mixture of CO and O₂ in a ratio of 2:1 and diluted by 20% ofN₂, the said operation being carried out in an autoclave, at thepressure of 4 atmospheres and the temperature of 45° C. The reaction wasprotracted for about 5 hours; the solution was then cooled to roomtemperature, subjected to a CO stream for 15 feet and analyzed bygas-chromatography and mass-spectroscopy. The gas-chromatographyexamination showed a solution amount of 3.75 g of ethylcarbonate, bycomparison with synthesis samples.

Other by-products were not observed in solution and CO₂ was absent inthe discharge gases. The same solution was again allowed to react in thesame conditions and for the same time.

By means of gas-chromatography analysis it was possible to observe adoubling of the carbonate concentration within the limits ofexperimental error.

What we claim is:
 1. A process for the preparation of esters of carbonicacid having the general formula: ##STR2## wherein R is a hydrocarbonradical selected from the class consisting of alkyl, aryl or cycloalkylradicals characterized in that an alcohol represented by the formula ROHin which R has the aforesaid meaning is reacted with carbon monoxide andoxygen in the presence of a catalyst consisting of cuprous chloridecomplexed with an organic ligand selected from the group consisting ofpyridine, dipyridyl, imidazole, phenanthroline alkyl or aryl,phosphines, dimethylsulfoxide, dimethylformamide, quinuclidine, CH₃ CN,C₆ H₅ CN, malonitrile, succinodinitrile and adiponitrile.
 2. A processaccording to claim 1 characterized in that the reaction is carried outin a solvent adapted to produce azeotropic mixtures with water andselected from:a. the same alcohol as that from which the ester isprepared; b. an organic compound adapted to be complexed by the employedcatalyst; c. an inert solvent; or a mixture of the said solvents.
 3. Aprocess according to claim 2 characterized in that the solvent is thereactant alcohol and the catalyst is put in as a complex.
 4. A processaccording to claim 1 characterized in that the temperature is preferablyselected in the range from -20° C. to +110° C.
 5. A process according toclaim 4 characterized in that the pressure is sufficient to ensure thatthe employed solvent will be kept in the liquid state at the processtemperature. .Iadd.
 6. A process for the preparation of esters ofcarbonic acid having the general formula: ##STR3## wherein R is ahydrocarbon radical selected from the class consisting of alkyl, aryl orcycloalkyl radicals characterized in that an alcohol represented by theformula ROH in which R has the aforesaid meaning is reacted with carbonmonoxide and oxygen in the presence of a catalyst consisting of cuprouschloride complexed with an organic ligand selected from the groupconsisting of pyridine, dipyridyl, imidazole, phenanthroline alkyl oraryl, phosphines, dimethylsulfoxide, dimethylformamide, quinuclidine,CO, CH₃ CN, C₆ H₅ CH, malonitrile, succinodinitrile andadiponitrile..Iaddend.